Binaphthyl units

Two different all-aromatic crowns have been reported ". In 1975, de Jong, Siegel and Cram reported the synthesis of a tri-binaphthyl system in which each oxygen was bound to a naphthalene ring, but aliphatic bridges were used to join the binaphthyl units. Relatives of this compound are discussed further in Sect. 3.13. The synthesis of this molecule (Structure 17, below) was not simple, but was relatively straightforward. An interesting feature of it was the use of ethyl chloroacetate followed by LAH reduction to attach ethyleneoxy units to the naphthol unit. 

Cram and his coworkers have pioneered the use of bis-binaphthyl crowns as chiral com-plexing agents for ammonium salts and amino acid salts. In these systems, the chiral binaphthyl unit provides a steric barrier within the macrocycle which allows discrimina-... 

The structural variations reported by Cram and coworkers relate to an appreciable extent to the various ancillary functions which have been appended to the binaphthyl units or elsewhere in the macrocyclic system. Enhancements of the chiral barrier or functionalization through arms has generally been effected at the 3-or 6-positions. These positions are adjacent to the hydroxyl group or directly across the second ring from it, respectively. 

The principal variations on the normal crown synthesis methods were applied in preparing mixed crowns such as those shown in Eq. (3.55) and in forming isomers of the dibinaphthyl-22-crown-6 systems. The latter has been discussed in Sect. 3.5 (see Eq. 3.21) . The binaphthyl unit was prepared to receive a non-naphthyl unit as shown in Eq. (3.57). Binaphthol was allowed to react with the tetrahydropyranyl ether or 2-chloroethoxyethanol. Cleavage of the THP protecting group followed by tosyla-tion of the free hydroxyl afforded a two-armed binaphthyl unit which could serve as an electrophile in the cyclization with catechol. Obviously, the reaction could be accomplished in the opposite direction, beginning with catechol". ... 

Lehn and his coworkers have prepared a number of chiral cryptands based upon the 2,2 -binaphthyl unit " . In a typical preparation, the binaphthyl units are treated with bromoacetic acid to form the phenoxyacetic acid derivatives which are then converted into the corresponding diacyl chlorides (75). Reaction of 15 with l,10-diaza-18-... 

If a bridged bis-crown is used instead of diaza-18-crown-6, the cryptand contains two macrorings facing each other (see Table 8.5). Note also that the 2,2 -binaphthyl unit has been used extensively by Cram and his coworkers to provide chirality to mono-cyclic systems as well (see Sect. 3.13). 

Optically active 1,1 -binaphthyl-based polymers incorporate chiral 1,17-binaphthyl units into the polymer backbone and possess stable main-chain... 

In 2001, Ahn et al. introduced a Mn(salen) possessing a structurally related binaphthyl unit, and also achieved high enantioselectivity in the epoxidation of conjugated olefins.103... 

Pu and co-workers incorporated atropisomeric binaphthols in polymer matrixes constituted of binaphthyl units, the macromolecular chiral ligands obtained being successfully used in numerous enantioselective metal-catalyzed reactions,97-99 such as asymmetric addition of dialkylzinc reagents to aldehydes.99 Recently, they also synthesized a stereoregular polymeric BINAP ligand by a Suzuki coupling of the (R)-BINAP oxide, followed by a reduction with trichlorosilane (Figure 10).100... 

Borner reported the synthesis of pyrophosphites 149 with chiral binaphthyl substituents [118]. The results showed that the Hg-binaphthyl unit was the best for the Rh-catalyzed hydrogenation of methyl (Z)-2-acetamidocinnamate (48% ee) and dimethyl itaconate (70% ee). 

In 1982, Yamashita reported the application of L-talopyranoside-based phos-phine-phosphinite ligand 165 (Fig. 27.15), and found that it induced low enan-tioselectivity (4.7-13% ee) in the hydrogenation of a-acetamidocinnamic acid [119]. Reetz introduced the phosphine-phosphonite ligand (151-153), which led to moderate enantioselectivity (52-88% ee) in the Rh-catalyzed hydrogenation of dimethyl itaconate [120]. The binaphthyl unit remained an essential element in the system. 

E. Ishow, A. Credi, V. Balzani, F. Spadola, L. Mandolini, A Molecular-level Plug/socket System. Electronic Energy Transfer from a Binaphthyl Unit Incorporated into a Crown Ether to an Anthracenyl Unit Linked to an Ammonium Ion , Chem. Eur. ]. 1999, 5, 984-989. 

A breakthrough was achieved with chiral phosphoramidite (S, R, i )-18, in which a C2-symmetric (S)-binaphthyl unit and a C2-symmetric (R, R)-bis-(l-phenylethyl)-amine unit are present (Scheme 7.10), resulting in the enantioselective catalytic 1,4-addition of Et2Zn to 2-cyclohexenone (6) with >98% ee [38]. 

The enantioselective oxidative coupling of 2-naphthol itself was achieved by the aerobic oxidative reaction catalyzed by the photoactivated chiral ruthenium(II)-salen complex 73. 2 it reported that the (/ ,/ )-chloronitrosyl(salen)ruthenium complex [(/ ,/ )-(NO)Ru(II)salen complex] effectively catalyzed the aerobic oxidation of racemic secondary alcohols in a kinetic resolution manner under visible-light irradiation. The reaction mechanism is not fully understood although the electron transfer process should be involved. The solution of 2-naphthol was stirred in air under irradiation by a halogen lamp at 25°C for 24 h to afford BINOL 66 as the sole product. The screening of various chiral diamines and binaphthyl chirality revealed that the binaphthyl unit influences the enantioselection in this coupling reaction. The combination of (/f,f )-cyclohexanediamine and the (R)-binaphthyl unit was found to construct the most matched hgand to obtain the optically active BINOL 66 in 65% ee. 

Racemic modifications may be resolved. There are very few examples of this approach having been employed successfully. The racemic cylic ether (RS)-36, which contains two CH2OCH2CO2H arms attached to the 3 and 3 positions on the axially chiral binaphthyl units, has been resolved (48-50, 93, 94) to optical purity in both its enantiomers by liquid-liquid chromatography using a chiral stationary phase of either (R)- or (S)-valine adsorbed on diatomaceous eaitii. Very recently, the optical resolution of crown ethers (/ S)-37 and (/ 5)-38, incorporating the elements of planar chirality in the form of a rron -doubly bridged ethylene unit, has been achieved (95) by HPLC on (+)-poly(triphenyl-methyl methacrylate). 

The monobenzhydiyl derivative of (S)-binaphthol has played an important role, not only in the synthesis of chiral bisbinaphthyl crown ether derivatives, for example, (55)-124, containing two different bridges between the two binaphthyl units, but also in the provision of an entry into the constimtionally isomeric derivative (5S)-125. Rational stepwise syntheses of macrocyles containing three binaphthyl units have been devised and applied to the synthesis of (SSS)-126 and (RSS)-127. Cleariy, in all these procedures, the C2 symmetry of the chiral building block restricts the number of products (to one ) and defines the symmetries of the macrocycles formed. 

Prelog and co-workers (160, 161) chose 9,9 -spirobifluorene as a starting material for synthesizing chiral crown ethers since (i) it has a more rigid carbon skeleton than the 1,1 -binaphthyl unit, and (ii) it can be substituted easily in the 2 and 2 positions by electrophilic reagents. Thus, the 2,2 -diacetyl derivative (Figure 19) obtained after a Friedel-Crafts on 9,9 -spirobifluoiene can be con-... 

Although a large number of biscrown ethers based on the binaphthyl nucleus have been synthesized (157) many of these have only been isolated and characterized as their racemic modifications. An exception is provided (157) by (SS)-152, obtained from the chiral bisbinaphthyl-22-crown-6 derivative in which one of the binaphthyl units carries chloromethyl groups (in the 3 and 3 positions) capable of reacting, in the presence of base, with tetraethylene glycol. 

The 22-crown-6 derivative (SS)-129 [incorporating (5)-binaphthyl units] with the (/ )-PhCH(C02Me)NH3 cation (210). 

The conjugation in the molecular wire may be disrupted or modulated to create systems with different properties. For example, a porphyrin Ceo donor-acceptor system linked with a conjugated binaphthyl unit, has a preference for the atropi-somer where the fullerene unit is closer to the porphyrin system, thus increasing the through space interactions [82]. The charge transfer process on a dyad containing a crown ether in the linker structure can be modulated by complexation/ decomplexation of sodium cations [83] but even more interesting is the construction of supramolecular systems where the donor and acceptor moieties are... 

Figure 6B.9. Role of the chiral binaphthyl unit of 15 in its asymmetric induction.

Further studies have shown that the torsional angle of the binaphthyl unit depends not only upon the steric demands of the two dendritic substituents, but... 

Measurements of molar rotation showed that this parameter is almost proportional to the number of chiral binaphthyl units and the molar rotation per binaphthyl unit varies only slightly. On catalysis of the Diels-Alder reaction of cyclopentadiene with 3-[(E)-but-2-enoyl]oxazolidin-2-one the branched catalysts 7 and 8 showed an approximately 25% higher reactivity than the monofunctional catalyst 6 however, the former led to just a slight improvement of ee and endo-selectivity compared to 6. It is thus inappropriate to speak of a dendritic effect on catalysis, although one does indeed exist in relation to the chiroptical properties. 

A new class of suitable optically active organocatalyst for enantioselective alkylations has recently been developed by Maruoka and co-workers [le, 33-37]. This catalyst is not based on an alkaloid-related quaternary ammonium salt but consists of a C2 -symmetric compound of type 29 (or derivatives thereof bearing other types of substituent on the 3,3 positions of the binaphthyl unit) [33, 34], In the presence... 

Ishow, E., Credi, A., Balzani, V., Spadola, F., Mandolini, L., A molecular-level plug/socket system Electronic energy transfer from a binaphthyl unit incorporated into a crown ether to an anthracenyl unit linked to an ammonium ion. Chem. Eur. J. 1999, 5, 984-989. 

However, the particular synthetic requirements in the preparation of conjugated polymers have thus far severely limited the number of similarly hierarchically structured examples. Pu et al. reported different types of conjugated polymers with fixed main-chain chirality containing binaphthyl units in their backbone which exhibited, for example, nonlinear optical activity or were used as enantioselective fluorescent sensors [42—46]. Some chirally substituted poly(thiophene)s were observed to form helical superstructures in solution [47-51], Okamoto and coworkers reported excess helicity in nonchiral, functional poly(phenyl acetylenejs upon supramolecular interactions with chiral additives, and they were able to induce a switch between unordered forms as well as helical forms with opposite helical senses [37, 52, 53]. 

Other swivel-cruciforms based on a 1,1 -binaphthyl core were prepared by Rajca et al. according to the procedure in Scheme 3.14 [44]. It is noteworthy that the solubility of the resulting materials in chloroform follows the order (S)-31 (2 X 10-1 mol L-1) > rac-31 (8 X 10-4 mol L-1) > 32 (2 X 10-5 mol L-1). Although the energies of the jt-jt transition for 31 and 32 were essentially identical, the extinction coefficient of 31 was approximately double that of the linear model compound 32. Both findings demonstrate the well-documented function of the binaphthyl unit to act as a conjugation barrier with little electronic communication between naphthalene units. 

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